Some designs of conventional down-the-hole hammers and fluid-operated percussion drill tools comprise an external cylinder or outer wear sleeve, within which is mounted an inner cylinder which in turn engages with a backhead assembly. A sliding reciprocating piston co-operates with the inner cylinder and backhead assembly, which when air pressure is supplied through the backhead assembly, acts with a percussive effect on a drill bit retained within a chuck on the outer wear sleeve.
Typically the inner cylinder is mounted co-axially within the outer wear sleeve. A sliding piston is mounted for reciprocating movement within the inner cylinder and the outer wear sleeve, to strike a hammer bit mounted for sliding movement in a chuck located at the forward end of the outer wear sleeve, in well known manner. A foot valve is positioned above the bit.
Our prior patent application Publication No. WO 2004/031530, discloses a down-the-hole hammer in which the bit has an elongate shank portion which at its upper end has an annular strike face (or anvil) against which the piston impacts to impart a percussive force to the bit. A lower end of the bit shank is formed externally with a plurality of splines, which are spaced around the circumference of the bit shank and extend in the axial direction. The splines slideably engage with complementary splines formed on the internal wall of an annular chuck. The chuck is screw-threadably connected to the bottom of the outer wear sleeve. The bit is retained in the hammer assembly by means of a bit retaining ring, which sits above the chuck and cooperates with an annular shoulder on the bit. This prevents the bit from falling out of the assembly in operation.
In operation the bit shank comes under forces due to the percussive action of the hammer, and rotational torque which is provided by the chuck. This imparts significant bending moments on the upper part of the bit shank increasing the risk of breakage of the shank due to cracking. Drill bits are very expensive to produce, and to recover if they are lost down the drilling hole. That this is a significant problem with the drill bits of conventional down-the-hole hammers is evidenced by the fact that there are a number of patents directed to means of retaining a broken-off bit within the bit assembly so as to prevent it falling down the drill hole. Examples of these patents are U.S. Pat. No. 5,065,827, U.S. Pat. No. 4,003,442, WO 96/15349, WO 98/05476, WO 03/062585, WO 03/062586. However, the inventions disclosed in these patents are directed to dealing with problems which occur after the bit shaft has fractured, and not to preventing the breakage in the first place.
Another disadvantage associated with conventional percussion drill tools, such as down-the-hole hammers, is that the bit has a long shank portion which is expensive to produce. The long shank portion is required in order to provide a splined shank portion of sufficient length to give enough support for transfer of rotational torque, and an area above the splines for retaining the bit. In conventional hammers, when the bit head or cutting face is worn out, the shank can often be in good condition but, because it is made integral with the cutting face, it must be discarded. The premature wearing out of the head/cutting face may occur where drilling is carried out in very abrasive rock or material which wears the tungsten carbide inserts in the cutting head. With many conventional hammers, there is a need to provide foot valves in the bit. The foot valve is required as an integral part of the functioning of the hammer i.e. when the piston is in the strike position, the bottom lift chamber is sealed by the bore of the piston and the outside of the footvalve. If this were not the case then the piston would not lift. The footvalve is prone to occasional breakage leading to down-time.
It is therefore desirable to provide a drill bit assembly for percussion drill tools, in which the length of the bit shank is substantially reduced in comparison to conventional percussion drill tools. However, when the bit is shortened in this way, retaining the bit within the chuck becomes more difficult. Traditional arrangements used for retaining long bit shanks in percussion drill tools, where a bit retaining ring sits above the chuck and cooperates with an annular shoulder on the bit, are not suitable for use with short stub shanks. This is because the maximum length of the drive chuck is limited by the length of the stub shank. As the chuck must include an upper screw thread portion for engagement with the outer wear sleeve, and a lower extension portion to protect the lower end of the outer wear sleeve from excessive wear, each of which must be sufficiently long to perform its intended purpose, it is desirable that the overall length of the chuck be maximised within the limit imposed by the shank length. It is therefore desirable to avoid an arrangement where a bit retaining ring must sit above the chuck, thereby further limiting the maximum length thereof.
Our granted European Patent No. 1 910 640 describes a drill bit assembly for fluid-operated percussion drill tools which overcomes a number of the problems discussed above. The assembly comprises a percussion bit having a head portion formed with an axially extending stub shank. The stub shank is provided with axially extending splines, which are slideably engageable with complementary splines formed on a drive chuck. Rotational drive from the chuck may be transmitted to the stub shank by means of the splines. Bit retaining means at the chuck are adapted for engagement with complementary retaining means at a spline portion of the stub shank to retain the stub shank in the drill bit assembly. Engagement means on the chuck are adapted for connecting the chuck to a drive means of the fluid-operated percussion drill tool.
This arrangement has a number of advantages over conventional systems. Because the means to retain the bit within the chuck has been moved to the splined portion of the stub shank, this assembly allows for a shortened shank and a maximised chuck length. In addition, splined support for transfer of rotational torque is provided both above and below the bit retaining means. A further advantage is that there is no requirement to have a footvalve in the bit. The footvalve and piston cooperation of earlier designs is replaced by the nose of the piston sealing in the bore of a bushing provided in the assembly.
However, there are also a number of disadvantages associated with the arrangement of EP 1 910 640. One such disadvantage is that the replacement of the footvalve with a piston nose which seals in the bore of a bushing is that the structure at the piston nose is relatively weak. In order to counteract this inherent weakness, it is desirable to increase the diameter ØB of the piston nose. However, the lift force applied to the piston in the strike position is given by the area of the pressure face on which the pressurised air acts, and is therefore proportional to the diameter of the piston ØA less the diameter of the piston nose ØB. It is desirable that the lift force be maximised within the hammer as the lift distance has a direct bearing on impact value and, in turn, drilling speed. Increasing the diameter of the piston nose to increase the strength of the piston results in a lower lift force, thereby detrimentally affecting performance of the drill.
It is therefore desirable to provide a hammer having a footvalve-less design in which piston strength and lift force are maximised.